attr: add patch to fix static installation

When attr is built static-only, it forgets to install its libattr.a
file, which leads to the build failure of packages such as 'acl' that
rely on attr.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>